The invention relates to a method and a system for transmitting data over switchable data networks, particularly Ethernet in the field of industrial installations, in which real-time critical and non real-time critical data are transmitted. The switchable data network has at least one wireless transmission link between at least two users, particularly a transmitter and a receiver.
Data networks enable communication between a plurality of users through networking, i.e., the connection of individual users among each other. Communication is the transmission of data between the users. The data to be transmitted are sent as data messages, i.e., the data are combined into a plurality of packets and are transmitted in this form to the corresponding receiver over the data network. Hence they are also referred to as data packets. The term data transmission as used in this document is completely synonymous with the aforementioned transmission of data messages or data packets. Networking per se is implemented, for example, in switchable high-performance data networks, particularly Ethernet, by inserting at least one coupling unit between two users, which is connected to both users. Each coupling unit may be connected to more than two users. If the user is integrated in a coupling unit, it is also possible that the coupling unit is only connected with another coupling unit or with another user, i.e., it may be a terminal. Each user is connected to at least one coupling unit but is not directly connected to another user. Users are, for example, computers, stored program controllers (SPCs) or other machines, which exchange electronic data with other machines and, in particular, process data of other machines. Transmission links between two coupling units, or between a coupling unit and a user, can be wire-bound or wireless. Wireless transmission of data may be effected, for example, by radio or infrared light. If the data network has only wireless transmission links it is called a wireless network, if it has both wireless and wire-bound transmission links, it is referred to as a heterogeneous network. In contrast to bus systems, in which each user can reach every other user of the data network directly via the data bus, switchable data networks only have point-to-point connections as transmission links. In other words, a user can only indirectly reach all the other users of the switchable data network by forwarding the data to be transmitted through one or more coupling units.
In distributed automation systems, e.g., in the field of drive engineering, specific data must reach and be processed by the intended users at specific times. One speaks of real-time critical data or data traffic because the failure of the data to arrive at the destination in time leads to undesirable consequences in the user. Successful real-time critical data traffic of the above-described type can be ensured in distributed automation systems according to IEC 61491, EN61491 SERCOS Interface—Brief Technical Description (http://www.sercos.de/deutsch/index_deutsch.htm).
It is likewise known in the related art to use a synchronously clocked communication system with equidistant characteristics in an automation system of this type. This is a system having at least two users that are interconnected via a data network for the purpose of mutually exchanging or transmitting data.
The data are exchanged cyclically in equidistant communication cycles, which are predetermined by the communication clock used by the system. Users are, for example, central automation devices, programming, configuration or control units, peripheral units, such as input/output modules, drives, actuators, sensors, stored program controllers (SPCs) or other control units, computers or machines that exchange electronic data with other machines and, in particular, process data of other machines. The term control units as used hereinafter means open loop or closed loop control units of any type. The data networks used are, for example, bus systems, e.g., field bus, Profibus, Ethernet, Industrial Ethernet, FireWire or PC-internal bus systems (PCI), etc.
Today, automation components (e.g., controllers, drives, etc.) generally have an interface to a cyclically clocked communication system. One processing level of the automation component (fast cycle) (e.g., position control in a control system, torque control of a drive) is synchronized to the communication cycle. This determines the communication clock. Other low-performance algorithms (slow cycle) (e.g., temperature controls) of the automation component can also only communicate with other components (e.g., binary switches for fans, pumps, etc.) by this communication clock, although a slower cycle would be sufficient. The use of a single communication clock for transmitting all the information created in the system places high demands on the bandwidth of the transmission link.
For communication on every process or automation level, prior art system components use only a single communication system or cycle (fast cycle) and its clock to transmit all relevant information. Data that are required only in the slow cycle can be transmitted in stages, e.g. using additional protocols, to limit the demands on the bandwidth. This results in additional software complexity in the automation components. Furthermore, both the bus bandwidth and the minimum possible communication cycle in the entire system are determined by the low-performance component.